This invention relates to defrost controls for a refrigerator, and more particularly, to an adaptive demand defrost control system which provides a variable interval between defrost operations which is based upon several factors, including the amount and duration of door openings and the length of previous defrost operations.
In general, in a refrigerator it is desirable to defrost only as often as is necessary to maintain an efficient cooling system. This objective dictates that a balance be struck between the competing considerations of system operation with a frosted evaporator, the energy consumed in removing a frost load from the evaporator and the acceptable level of temperature fluctuation within the refrigerated compartments caused by a defrosting operation.
A successful attempt at meeting this objective is shown and described in U.S. patent application Ser. No. 155,154, now U.S. Pat. No. 4,327,557, filed May 30, 1980, entitled "Adaptive Defrost Control System" and assigned to the assignee of this application. The system disclosed therein takes into account the number and duration of freezer and fresh food door compartment openings, the duration of the previous defrosting operation, and the total accumulated compressor run time since the previous defrost operation. In general, defrosting is provided at variable intervals as determined by a weighted accumulation of the number and duration of freezer and fresh food door openings, with the weighting functions being adaptably controlled as a function of the time required to perform the previous defrost operation.
The control disclosed in the above application stores a count which is decremented by the weighting functions during a door-open interval. The count is decremented at a first constant rate during a first predetermined period of time that the fresh food door is open, and is decremented at a second constant rate thereafter. The count is decremented at a third constant rate during an initial predetermined period of time that the freezer door is open, and a fourth constant rate thereafter.
The rates of decrementing the count are determined by comparing the measured length of a defrosting operation against a desired defrost length. In many instances, the comparison of the measured defrost length with the desired defrost length operates to change the length of the interval before the next defrost operation, in turn forcing the next succeeding defrost length toward the desired value.
While the defrost control described above has been successful in implementing efficient control of a defrost heater, it has been found that efficiency can be further increased if, in addition to the factors utilized by the above described defrost control, the evaporator temperature is considered as a factor in determining the length of a defrost interval.
Generally, it has been found that there is little or no correlation between the duration of a defrost operation and the amount of frost which has actually been removed from the evaporator during the defrost operation. This is due to the fact that the measured length of a defrost operation is not only dependent upon the amount of frost on the evaporator coil, but is also strongly dependent upon the temperature of the evaporator at the time the defrost operation is initiated. Since the defrost control disclosed in the above-mentioned patent utilizes the length of a defrost operation as a factor in determining the duration of the next defrost interval, the defrost control may provide less-than-optimal defrost operation if the temperature of the evaporator is not considered.
Moreover, it has been found that the decrementing of the count at constant rates during the time the fresh food door is open does not result in an entirely accurate representation of the amount of frost which has formed on the evaporator due to the moisture introduced into the refrigerator while the door is open. Again, this may result in a less-than-optimal defrost interval.
Furthermore, it has been found desirable to incorporate control of a humidity-dependent apparatus, such as an anti-sweat heater, in accordance with the ambient humidity to which the refrigerator is exposed. Reliable humidity sensors are, however, relatively expensive and impractical for use on household refrigerators and the like.